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Sebastien Bihorel

2018-Oct-30 20:18 UTC

[R] Question about function scope

Thanks Duncan for your quick reply.

Ideally, I would want bar1 and bar2 to be independent functions, because they
are huge in actuality and, as the actual foo function grows, I may end up with
10 different bar# functions. So I would like to separate them from foo as much
as possible.


----- Original Message -----
From: "Duncan Murdoch" <murdoch.duncan at gmail.com>
To: "Sebastien Bihorel" <sebastien.bihorel at cognigencorp.com>,
r-help at r-project.org
Sent: Tuesday, October 30, 2018 4:13:05 PM
Subject: Re: [R] Question about function scope

On 30/10/2018 3:56 PM, Sebastien Bihorel wrote:> Hi,
> 
>  From the R user manual, I have a basic understanding of the scope of
function evaluation but have a harder time understanding how to mess with
environments.
> 
> My problem can be summarized by the code shown at the bottom:
> - the foo function performs some steps including the assignment of default
values to 3 objects: x, y, z
> - at some point, I would like to call either the bar1 or bar2 function
based upon the value of the c argument of the foo function. These functions
assign different values to the x, y, z variables.
> - then foo should move on and do other cool stuff
> 
> Based upon default R scoping, the x, y, and z variables inside the bar1 and
bar2 functions are not in the same environment as the x, y, and z variables
created inside the foo function.
> 
> Can I modify the scope of evaluation of bar1 and bar2 so that x, y, and z
created inside the foo function are modified?
> 
> PS:
> - I know about "<<-" but, in my real code (which I cannot
share, sorry), foo is already called within other functions and x, y, and z
variables do not exist in the top-level environment and are not returned by foo.
So "<<-" does not work (per manual: " Only when <<-
has been used in a function that was returned as the value of another function
will the special behavior described here occur. ")
I haven't looked up that quote, but it is likely describing a situation 
that isn't relevant to you.  For you, the important part is that bar1 
and bar2 must be created within foo.  They don't need to be returned 
from it.

So my edit below of your code should do what you want.

foo <- function(a=1, b=2, c=0){

   bar1 <- function(){
     x <<- 1
     y <<- 1
     z <<- 1
     cat(sprintf('bar1: x=%d, y=%d, z=%d\n', x, y, z))
   }

   bar2 <- function(){
     x <<- 2
     y <<- 2
     z <<- 2
     cat(sprintf('bar2: x=%d, y=%d, z=%d\n', x, y, z))
   }

   # some setup code
   dummy <- a + b
   x <- y <- z <- 0

   # here is my scope problem
   if (c==1) bar1()
   if (c==2) bar2()

   # some more code
   cat(sprintf('foo: x=%d, y=%d, z=%d\n', x, y, z))

}

foo(c=0)
foo(c=1)
foo(c=2)

I get this output:

 > foo(c=0)
foo: x=0, y=0, z=0
 > foo(c=1)
bar1: x=1, y=1, z=1
foo: x=1, y=1, z=1
 > foo(c=2)
bar2: x=2, y=2, z=2
foo: x=2, y=2, z=2

Duncan Murdoch

Duncan Murdoch

2018-Oct-30 20:47 UTC

head link

[R] Question about function scope

On 30/10/2018 4:18 PM, Sebastien Bihorel wrote:> Thanks Duncan for your quick reply.
> 
> Ideally, I would want bar1 and bar2 to be independent functions, because
they are huge in actuality and, as the actual foo function grows, I may end up
with 10 different bar# functions. So I would like to separate them from foo as
much as possible.
If that's the case, then I think the second solution (passing around 
environments) is a really bad idea.  Functions should not have side 
effects because it makes them harder to understand.  Modifying local 
variables in some other function is a really dangerous side effect, 
especially if both functions are big, because they are already hard to 
understand.

If you really have more callers for bar1() than just foo(), it is even 
worse.

So I'd suggest having bar1 and bar2 return the new values in a list, and 
in foo(), explicitly extract the values you want from the list.  Then if 
in the future you decide that bar1 should also return a 4th value w, or 
you want to rename x to something more meaningful, you don't need to 
check your other foo functions to see if x and w are used in the same 
way in them.  They'll just ignore w if it isn't relevant to them.  Your 
foo() code becomes something like this:

   x <- y <- z <- 0

   # here is my scope problem
   result <- list(x = x, y = y, z = z) # c == 0 case
   if (c==1) result <- bar1()
   if (c==2) result <- bar2()
   x <- result[["x"]]
   y <- result[["y"]]
   z <- result[["z"]]

Duncan Murdoch

> 
> 
> ----- Original Message -----
> From: "Duncan Murdoch" <murdoch.duncan at gmail.com>
> To: "Sebastien Bihorel" <sebastien.bihorel at
cognigencorp.com>, r-help at r-project.org
> Sent: Tuesday, October 30, 2018 4:13:05 PM
> Subject: Re: [R] Question about function scope
> 
> On 30/10/2018 3:56 PM, Sebastien Bihorel wrote:
>> Hi,
>>
>>   From the R user manual, I have a basic understanding of the scope of
function evaluation but have a harder time understanding how to mess with
environments.
>>
>> My problem can be summarized by the code shown at the bottom:
>> - the foo function performs some steps including the assignment of
default values to 3 objects: x, y, z
>> - at some point, I would like to call either the bar1 or bar2 function
based upon the value of the c argument of the foo function. These functions
assign different values to the x, y, z variables.
>> - then foo should move on and do other cool stuff
>>
>> Based upon default R scoping, the x, y, and z variables inside the bar1
and bar2 functions are not in the same environment as the x, y, and z variables
created inside the foo function.
>>
>> Can I modify the scope of evaluation of bar1 and bar2 so that x, y, and
z created inside the foo function are modified?
>>
>> PS:
>> - I know about "<<-" but, in my real code (which I
cannot share, sorry), foo is already called within other functions and x, y, and
z variables do not exist in the top-level environment and are not returned by
foo. So "<<-" does not work (per manual: " Only when
<<- has been used in a function that was returned as the value of another
function will the special behavior described here occur. ")
> 
> I haven't looked up that quote, but it is likely describing a situation
> that isn't relevant to you.  For you, the important part is that bar1
> and bar2 must be created within foo.  They don't need to be returned
> from it.
> 
> So my edit below of your code should do what you want.
> 
> foo <- function(a=1, b=2, c=0){
> 
>     bar1 <- function(){
>       x <<- 1
>       y <<- 1
>       z <<- 1
>       cat(sprintf('bar1: x=%d, y=%d, z=%d\n', x, y, z))
>     }
> 
>     bar2 <- function(){
>       x <<- 2
>       y <<- 2
>       z <<- 2
>       cat(sprintf('bar2: x=%d, y=%d, z=%d\n', x, y, z))
>     }
> 
>     # some setup code
>     dummy <- a + b
>     x <- y <- z <- 0
> 
>     # here is my scope problem
>     if (c==1) bar1()
>     if (c==2) bar2()
> 
>     # some more code
>     cat(sprintf('foo: x=%d, y=%d, z=%d\n', x, y, z))
> 
> }
> 
> foo(c=0)
> foo(c=1)
> foo(c=2)
> 
> I get this output:
> 
>   > foo(c=0)
> foo: x=0, y=0, z=0
>   > foo(c=1)
> bar1: x=1, y=1, z=1
> foo: x=1, y=1, z=1
>   > foo(c=2)
> bar2: x=2, y=2, z=2
> foo: x=2, y=2, z=2
> 
> Duncan Murdoch
>

Neal Fultz

2018-Oct-31 01:28 UTC

head link

[R] Question about function scope

I'm going to n-th Duncan's recommendations to try to keep your functions
small and try not to mess with environments.

... So I'm ashamed that I wrote the following - I apologize in advance.

But as an intellectual exercise, we can cannibalize the code for `dynGet`
and create a `dynSet` function which mirrors it:

---

dynSet <- function (x, value, ifnotfound = stop(gettextf("%s not
found",
sQuote(x)),
                               domain = NA), minframe = 1L, inherits FALSE)
{
  n <- sys.nframe()
  myObj <- structure(list(.b = as.raw(7)), foo = 47L)
  while (n > minframe) {
    n <- n - 1L
    env <- sys.frame(n)
    r <- get0(x, envir = env, inherits = inherits, ifnotfound = myObj)
    if (!identical(r, myObj)) {
      assign(x, value, envir = env)
      return(NULL)
    }
  }
  ifnotfound
}

### Note that in bar1 / bar2, you can't use x,y,z - dynSet will match the
local variable instead of the calling frame.

bar1 <- function(){
  x1 <- 1
  y1 <- 1
  z1 <- 1
  cat(sprintf('bar1: x=%d, y=%d, z=%d\n', x1, y1, z1))
  dynSet("x", x1)
  dynSet("y", y1)
  dynSet("z", z1)

}

bar2 <- function(){
  x2 <- 2
  y2 <- 2
  z2 <- 2
  cat(sprintf('bar2: x=%d, y=%d, z=%d\n', x2, y2, z2))
  dynSet("x", x2)
  dynSet("y", y2)
  dynSet("z", z2)

}

foo <- function(a=1, b=2, c=0){

  # some setup code
  dummy <- a + b
  x <- y <- z <- 0

  # here is my scope problem
  if (c==1) bar1()
  if (c==2) bar2()

  # some more code
  cat(sprintf('foo: x=%d, y=%d, z=%d\n', x, y, z))

}

foo(c=0)
foo(c=1)
foo(c=2)




---

Please don't actually do this though.

I think the most educational part of this is the dynGet/dynSet function
itself - if you understand the various functions it uses, you can do pretty
much anything in R.


But again, you shouldn't.





On Tue, Oct 30, 2018 at 1:51 PM Duncan Murdoch <murdoch.duncan at
gmail.com>
wrote:
> On 30/10/2018 4:18 PM, Sebastien Bihorel wrote:
> > Thanks Duncan for your quick reply.
> >
> > Ideally, I would want bar1 and bar2 to be independent functions,
because
> they are huge in actuality and, as the actual foo function grows, I may end
> up with 10 different bar# functions. So I would like to separate them from
> foo as much as possible.
>
> If that's the case, then I think the second solution (passing around
> environments) is a really bad idea.  Functions should not have side
> effects because it makes them harder to understand.  Modifying local
> variables in some other function is a really dangerous side effect,
> especially if both functions are big, because they are already hard to
> understand.
>
> If you really have more callers for bar1() than just foo(), it is even
> worse.
>
> So I'd suggest having bar1 and bar2 return the new values in a list,
and
> in foo(), explicitly extract the values you want from the list.  Then if
> in the future you decide that bar1 should also return a 4th value w, or
> you want to rename x to something more meaningful, you don't need to
> check your other foo functions to see if x and w are used in the same
> way in them.  They'll just ignore w if it isn't relevant to them. 
Your
> foo() code becomes something like this:
>
>    x <- y <- z <- 0
>
>    # here is my scope problem
>    result <- list(x = x, y = y, z = z) # c == 0 case
>    if (c==1) result <- bar1()
>    if (c==2) result <- bar2()
>    x <- result[["x"]]
>    y <- result[["y"]]
>    z <- result[["z"]]
>
> Duncan Murdoch
>
>
> >
> >
> > ----- Original Message -----
> > From: "Duncan Murdoch" <murdoch.duncan at gmail.com>
> > To: "Sebastien Bihorel" <sebastien.bihorel at
cognigencorp.com>,
> r-help at r-project.org
> > Sent: Tuesday, October 30, 2018 4:13:05 PM
> > Subject: Re: [R] Question about function scope
> >
> > On 30/10/2018 3:56 PM, Sebastien Bihorel wrote:
> >> Hi,
> >>
> >>   From the R user manual, I have a basic understanding of the
scope of
> function evaluation but have a harder time understanding how to mess with
> environments.
> >>
> >> My problem can be summarized by the code shown at the bottom:
> >> - the foo function performs some steps including the assignment of
> default values to 3 objects: x, y, z
> >> - at some point, I would like to call either the bar1 or bar2
function
> based upon the value of the c argument of the foo function. These functions
> assign different values to the x, y, z variables.
> >> - then foo should move on and do other cool stuff
> >>
> >> Based upon default R scoping, the x, y, and z variables inside the
bar1
> and bar2 functions are not in the same environment as the x, y, and z
> variables created inside the foo function.
> >>
> >> Can I modify the scope of evaluation of bar1 and bar2 so that x,
y, and
> z created inside the foo function are modified?
> >>
> >> PS:
> >> - I know about "<<-" but, in my real code (which I
cannot share,
> sorry), foo is already called within other functions and x, y, and z
> variables do not exist in the top-level environment and are not returned by
> foo. So "<<-" does not work (per manual: " Only when
<<- has been used in a
> function that was returned as the value of another function will the
> special behavior described here occur. ")
> >
> > I haven't looked up that quote, but it is likely describing a
situation
> > that isn't relevant to you.  For you, the important part is that
bar1
> > and bar2 must be created within foo.  They don't need to be
returned
> > from it.
> >
> > So my edit below of your code should do what you want.
> >
> > foo <- function(a=1, b=2, c=0){
> >
> >     bar1 <- function(){
> >       x <<- 1
> >       y <<- 1
> >       z <<- 1
> >       cat(sprintf('bar1: x=%d, y=%d, z=%d\n', x, y, z))
> >     }
> >
> >     bar2 <- function(){
> >       x <<- 2
> >       y <<- 2
> >       z <<- 2
> >       cat(sprintf('bar2: x=%d, y=%d, z=%d\n', x, y, z))
> >     }
> >
> >     # some setup code
> >     dummy <- a + b
> >     x <- y <- z <- 0
> >
> >     # here is my scope problem
> >     if (c==1) bar1()
> >     if (c==2) bar2()
> >
> >     # some more code
> >     cat(sprintf('foo: x=%d, y=%d, z=%d\n', x, y, z))
> >
> > }
> >
> > foo(c=0)
> > foo(c=1)
> > foo(c=2)
> >
> > I get this output:
> >
> >   > foo(c=0)
> > foo: x=0, y=0, z=0
> >   > foo(c=1)
> > bar1: x=1, y=1, z=1
> > foo: x=1, y=1, z=1
> >   > foo(c=2)
> > bar2: x=2, y=2, z=2
> > foo: x=2, y=2, z=2
> >
> > Duncan Murdoch
> >
>
> ______________________________________________
> R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
> http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>
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R help - Oct 2018 - Question about function scope

[R] Question about function scope

[R] Question about function scope

[R] Question about function scope